Radio-amplifying circuits



Dec. 19, 1933.

W, J. POLYDOROFF RADIO AMPLIFYING CIRCUITS Filed Aug. 26. 1929 3Sheets-Sheet 2 l I I I I l [III O +/O +20 +30 OFF RESONANCE I: I II :I'I I II I I K/ZOCJ/CZES OFF RESO/V/V/VCF k I I lllll A TTORNE Y PatentedDec. 19, 1933 PATENT OFFICE 1,940,228 RADIO-AMPLIFYING CIRCUITS WladimirJ. Polydorofl, Chicago, 111., asslgnor to Johnson Laboratories, Inc.,Chicago, 111., a corporation of Illinois Application August 26, 1929.Serial No. 388,289 19 Claims. (01. 179-171) The invention relates to theuse of iron or other magnetic materials in radio-frequency and otheralternating current circuits. More particularly, the present inventioncontemplates the use of I such materials in tuned radio-frequencyamplifying circuits, although the particular new instrumentalities to bedescribed and claimed are not limited in their advantageous applicationto this particular class of apparatus.

As is well understood in the radio art, a tuned radio-frequencyamplifying circuit consists essentially of a relay device such as athermionic amplifying tube, and an electrically connected resonantcircuit consisting of inductance and capacity. This resonant circuit isthe portion of the radio-amplifying circuit in which the property ofselectivity resides, and it may therefore be referred to as theselective circuit, as distinguished from the complete amplifying circuitincluding the tube or relay.

Hitherto, many attempts were made to employ iron cores in thetransformers used in radio frequency, but owing to great lossesintroduced by the iron as then employed, efficiency was generallyimpaired, so that air-core coils and transformers are now exclusivelyused in the cases where relatively high-frequency oscillations, eithermodulated or not by voice frequencies, are to be selected and amplifiedby thermionic tubes and associated circuits.

The invention will be better understood if reference is made to theaccompanying drawings where Figure 1 represents a general circuitemploying thermionic tubes and embodying the present invention;

Figure 2 is a diagram showing the amplification of a single stage ofsuch a system;

Figure 3 is a diagram showing the resistance variations of difierenttransformers or coils;

Figures 4, 5, -6, '7 show various modifications of the presentinvention;

Figure 8 shows one application of the present invention;

Figure 9 shows the amplification curve of one stage of the circuit, ofFigure 8;

Figures 10, 11, 12 and 13 show selectivity curves of amplifiers ofvarious designs, and

Figures 14 and 15 show modifications of the invention.

Figure 16 shows a tuning device including correlated elements.

It is an usual practice to employ several stages of radio-frequencyamplifiers in cascade, such as shown on Figure 1, having variableinductances ill REISSUED mac 5 was or capacities, or both, in tuningcircuits, so as to cover a certain band of frequencies, say from 1,500

to 550 kilocycles. Such a wide range of frequencies renders thoseamplifying circuits inefficient and different in operation at differentfrequencies.

It is the object of the present invention to improve fidelity,selectivity and efiiciency and to secure uniformity of amplification.

Mathematical analysis (Victor G. Smith, Proc.

I. R. E. vol. 15, No. 6, June 1927) shows that amplification of tunedradio-frequency circuits is represented by the ratio of output to inputvoltages:

n 1 l 2 Vi w/ l Vzancl W 02 I where w-2rrfi providing the circuit istuned to the resonance, and optimum coupling between primary andsecondary circuits is obtained for every given frequency. These formulaeprove that, in the case of fixed value of inductance L2 in order to keepthe amplification constant, the resistance R2 of the tuned circuit,which is chiefiy composed of the coils resistance, should changeproportionately to the square of the frequency.

In the case of a fixed value of mutual inductance, M, the amplificationdecreases with increase of tuning capacity as represented by the curve aof Figure 2, wherein mutual inductance is adjusted for thehigh-frequency end of the frequency band. Mutual inductance can bechosen to be at the optimum value somewhere in the middle of thefrequency band as represented by the curve b of said figure. In thiscase, the circuits, being over-coupled at high frequency andunder-coupled at low frequency, produce non-uniform amplification. Theabove formulae also prove that in order to keep the mutual inductance atits optimum value throughout the frequency band, the resistance R2should be so changed as still to be proportional to the square ofthefrequency.

Actual measurements of the resistance of an air coil or transformer,show that the coil changes its resistance with frequency changes, butnot as much as would be required to keep the amplification ccnstant andthe mutual inductance at optimum. Curve a of Figure 3, shows the re-.sistance for a certain air coil, measured at different frequenciesgwhilecurve b represents the resistance required to maintain said resistanceproportional to the square of frequency.

One object of this invention is to provide radiofrequency circuits withtransformers and coils which will not be subject to the describeddeficiencies of air coils, to thereby attain even amplification withoptimum mutual inductance. It has been found that the use offinely-powdered iron in the field of an air coil, will change thevariation of the resistance of the coil relatively to variations infrequency.

A low-resistance air coil was chosen, and its inductance was materiallyaugmented by powdered iron disposed within said coil. The actualmeasurements of resistance throughout the entire range of frequenciesutilized, shown by the curve I) of Figure 3, indicate that theresistance changed approximately nine times while the frequency changedapproximately three times. Numerous measurements have verified the factthat the resistance of iron-core coils remains proportional to thesquare of the frequency, despite frequency changes.

Transformers having powdered iron cores, were connected with thermionictubes and other elements of the circuit of Figure 1, and it was foundthat the amplification curve remained substantially even throughout theentire range of frequencies, as represented by the curve '0 of Figure 2.Thus, a close agreement with theoretical considerations was established,that is to say, that when resistance varies as the square of thefrequency, the amplification of a tuned circuit remains constant and themutual inductance adjusts itself to optimum value.

The resistances of transformers having powdered-iron cores, beingextremely small at 550 k. c. and the lower frequencies, it is possibleto build transformers and coils designed for those low frequencieshaving an L/R ratio much greater than is practical if ordinary air coilsare employed. Furthermore, it is possible to reduce the windings andthereby minimize the size of radio-frequency transformers, thuseffectuating a saving of space.

There may be several embodiments of my ironcore coils and transformers,the simplest embodiment being shown in Figure 4 wherein primary andsecondary coils 1, 2 are wound on an insulated tube 3, and powdered iron4 is packed inside the tube 3, and the ends 5 of the tube are sealed.

Figure 5 shows schematically a binocular coil 6 wound on insulated tubes7, portions 8 of the powdered iron being separated by thin paper discs9, to prevent readjustment of its particles and consequent packing andslight change in inductance.

Iron cores can also be made by mixing various adhesive and insulatingcompounds with powdered iron, and giving these cores the desired shapewith or without pressure. Also, certain other insulators, such as wax,paraffine and min eral oils, may be used. Substances when melted andmixed with iron particles, while hot manifest very small losses but,when cold and solidified, increase the conductivity of iron coresthousands of times and establish easy paths for eddy currents at highfrequencies, resulting in a considerable increase in radio-frequencyresistance of the coils equipped with such cores.

However, a coil having either a melted or an unmelted core of wax,without powdered iron, developed substantially no resistance in thecoil, while, when powdered iron was mixed with the wax of said core, theresistance in said core was 5 ohms, if the wax was melted, and was 50ohms, if the wax was solidified. Also, when the core comprised the samemass and quality of powdered iron the resistance of the coil was 5 ohms.

Other insulators preferably of elastic nature,

such as rubber, natural and synthetic gums and certain varnishes mixedwith iron and pressed together, will maintain insulating films betweenthe iron particles and, therefore, reduce resultant radio-frequencylosses.

The definition iron used in the specification should be applied to anyother metal or alloy having magnetic properties, such as silicon-iron,permalloy and nickel-iron. Various powdered metals were tested forradio-frequency transformers and inductances, and it was found that forthe most satisfactory results iron reduced by hydrogen, the particles ofwhich had been sifted through a screen of 300 meshes to the inch, shouldbe used for frequencies between 1500 to 1000 k. 0., but" that powderscontaining coarser particles may be used for frequencies below 1000 k.c., and also that the fineness and the force of compression govern theradio-frequency resistance of the coil. Iron produced by hydrogen in theordinary way contains particles of various sizes some of which may betoo large for use in the production of cores suitable for use in radiofrequency circuits. It is, therefore, necessary to eliminate the largestparticles. The insulating coat of oxide usually present on the surfacesof iron particles, helps to reduce eddy-current losses. When siliconiron powder is used it is practical to chemically treat the powder witha phosphoric acid solution which creates an insulating film.

When, in this specification, powdered iron is referred to, I mean eitherincoherent masses of finely-divided iron, or masses of finely-dividediron compressed into bodies in which the individual particles are heldtogether by an insulating 110 binder, but the degree of compressionshould not be so great as to cause the particles of powdered iron totouch each other and thus exclude the insulating material which shouldseparate them.

To obtain maximum gain in inductance, thus due to the iron cores, thelength of a coil should be preferably twice its diameter, and the wireshould be space-wound, such arrangement being shown in Figure 6, wherein10 indicates a spacewound wire, and 11 indicates the powdered iron core.The inductance may be further augmented if the iron core extends aroundthe outside of the coil in the form of cylinder 12. Such arrangement ofiron completely closes the magnetic lines around the coil, forming anastatic coil. 125 When iron cores are employed in the coils of theclosed or semi-closed magnetic field type, the original astaticproperties of such coils are great- 1y enhanced. While the powdered-ironcore substantially doubles the individual primary and 130 secondaryinductances, it increases the mutual inductance between the windingsfour or five times.

This phenomenon is especially advantageous when a very tight coupling isrequired, or when 135 long solenoids are employed for transformation.

Another object of the present invention is to tune a radio-frequencycircuit in a new, simple and efficient manner, a movable powdered ironcore disposed in the field of a coil being employed 140 for tuningpurposes. Depending on the amount of the iron powder inserted in theform of a core, the self-inductance of a coil may be increased four andeven six times with a resulting increase 14 in radio-frequencyresistance. 5

Figure 7 shows a binocular transformer 13, having a core 14 which can bemoved in and out to obtain variations of self-inductance and mutualinductance. This device is capable of tuning the 150 circuit to .adesired frequency when connected with tubes and associated circuits suchas shown in Figure 1, or such as are shown in Figure 8 wherein aradio-frequency choke amplifier is represented. In this figure are shownthermionic tubes, 15 and 16, acting as radio-frequency ampliflers, adetector tube 17, choke coils 18 having powdered-iron movable cores 19,suitable resistances 20, coupling condensers 21, a telephonic re-'ceiver 22, and a plate battery 23, filament-heating means being omittedfor simplicity. Inthis system it is possible to work the amplifier atits full efiiciency throughout a given range of frequencies by movingiron cores inward or outward. The amplification per stage is shown bycurve a of Figure 9. Core movements can be made simultaneous with themovements of the input selector, which selector is usual to secure thenecessary selectivity for wave lengths of different broadcastingstations.

When the selecting of a signal accompanies amplification, as in thecascade radio-frequency amplifier shown in Figure 1, the ability toselect is called selectivity of tuned circuits. In an ordinary air-coretransformer associated with a thermionicamplifier, selectivity varieswith frequency. Actual measurements, made for a circuit having L-180microhenrys variable capacity from 10 to 500 micro-microfarads and totalresistance of the circuit varying from 7 to 4 ohms, are shown in Figure10, wherein, the amplification is plotted against kilocycles off ofresonance. Curve a is taken at a frequency of 1400 k. 0., curve b at1000 k. c., and curve 0 at 600 k. 0. Vertical lines d and g are drawn at10 k. 0. off of resonance and represent interfering adjacent stations.Curve a shows inadequate selectivity due to high resistance of thecircuit and to lack of capacity. As frequency decreases, .the resistancedecreases the result being sharpening of the curves b and 0 representingselectivity. By decreasing the inductance and the losses in the circuitand increasing the initial amount of tuning capacity, it is possible toobtain sharper curves as shown in Figure 11. These three curves a, b and0, represent a very satisfactory selectivity, the amplitudes, as shown,being different because of fixed mutual inductance. The curves of Figure12 represent the selectivity obtained from a transformer, equippedwith afixed powdered-iron core as hereinbefore described.

As the radio frequency signal is usually modulated by voice frequencies,three frequencies is, fo-fv, and fo+fv, Where in is carrier frequency,,fv is voice frequency, have to be passed through a selective circuitwith substantially equal intensities in order to avoid distortion. Tosecure intelligible audio signals, it is usual to modulate carrierfrequency with voice frequencies ranging from 0 to 5000 cycles, and,therefore, selective circuits should be capable of passing a band offrequencies 10 kilocycles wide, to obtain the fidelity of reproduction.Two vertical dotted lines it and k in Figures 10, 11 and 12, representthe limits of audio-frequency modulations to be passed through eachselective circuit. Referring back to Figure 10, one can easily perceivethat curve a shows almost perfect fidelity, curve b shows slightlydistorted fidelity, the extreme side bands being attenuated about 15% ascompared with carrier frequency, and curve 0 shows attenuation of itsside bands and, therefore, introduces distortion. Curve (1. of Figure 11shows good fidelity, but curves b and 0 show distortion.

Comparison of these curves with curves obtained by the use ofpowdered-iron core transformers, shows that all of the curves of Figure12 have very good fidelity, and poor selectivity. It is thereforeessential for good selectivity and ndelity, throughout the entirefrequency range, to combine the selectivity of a low-loss high-capacitycircuit at high frequency with the selectivity and fidelity obtained bypowdered-iron cores at low frequency.

An amplifier was constructed with low-loss, low-inductance coils and amovable powdered-iron core and included in the circuit shown byFigure 1. Tuned to high-frequency signals, such as 1500-1200 k. c., saidcore was entirely withdrawn and the amplifier then developed thecharacteristics of the curve a of Figure 11. From any high-frequencybetween 1500 and 1200 k. c., to 550 k. c., said core was gradually movedinside the coil, and, due to increases of self and mutual inductancesand of radio-frequency resistance; the curves became broad as shown bycurves b and c of Figure 12. The resultant group of curves, taken atthree different frequencies, is separately represented in Figure 13 andshows substantially the same selectivity and fidelity throughout theentire range of frequencies.

As it is customary to express the selective properties in terms of bandwidths in kilocycles at half amplitude, I have chosen to call thisquantity the selectance and have used this word in this sense in theappended claims. As determined from Figure 13, the selectance is of theorder of 30 k. c., slightly varying at three investigated frequencies.Theoretical analysis of a resonant circuit indicates that selectanceexpressed in band width is directly proportional to Where R and L are,respectively, resistance and inductance of the circuit, selectance beingsomewhat impaired by the effects of the plate electrode of an associatedthermionic amplifier,should an amplifying stage be employed. The factthat the selectances are substantially equal at three investigatedfrequencies, indicates that age of said amplifier being fed to asucceeding thermionic. amplifier through a condenser 28 and a gridresistance 27. Figure 15 shows essentially the same circuit 24 at 29,with the exception that output voltage is fed through an additionalwinding 30 unitarily coupled, to the inductance of the tuned circuit 29.

The amplification may be theoretically expressed as where a is theamplification factor and R1 the plate resistance of the thermionicamplifier, and

R0 the dynamic resistance of the circuit 24, in resonance equal to ofthe circuit. As the inductance L is varied for tuning, the resistance Rof the circuit, resulting from the coil resistance and iron core losses.should proportionately vary so as to maintain with the resultant R0 andconstant.

In case of simultaneous variation of capacity and inductance, theamplification of such a circuit can be represented by the above formulaemodified as follows:

is dynamic resistance of circuit in resonance. In addition to the changeof value of L2 and C2, B2 is also varied by the movement of the ironcore. By properly designing the low-loss coils, and properly choosingthe quantity, fineness of and pressure on the iron, as well as adjustingthe movement of the iron core in conjunction with other tuning elements,amplification may be kept constant.

Using loose coupling between the primary and secondary circuits, it wasfound favorable to vary the mutual inductance to a very large extent, sothat at high frequencies the primary and secondary circuits wereunder-coupled while at low frequencies they were over-coupled, thesevariations being produced automatically by the said movements of theiron core.

When movable iron cores are used for tuning purposes in conjunction withother variable devices, such as variable condensers, variableinductances, variometers and the like, the movements of iron coresshould be correlated with the movements of said variable devices. It ispreferable to so adjust the cores that, at the higher frequencies, theiron is kept away from the coils. As the frequency decreases, the ironcores should be gradually inserted into the coils, with a constant or anaccelerated speed, depending on the design of other tuning devicesemployed.

Figure 16 shows a selective device embodying a movable iron-core whichcan be used in connection with amplifying circuits, such as shown inFigures 1, l4 and 15. A variable condenser 31 having stationary plates32 and rotary plates 33, carries a grooved cam 34 firmly connected withthe shaft 35. A lever 36, engaging the cam groove, actuates a verticalrod 37 to which a powderediron core 38 is fixed. A transformer 39, whichmay be an impedance coil, telescopically receives said core 38. When thecondenser plates are rotated for tuning purposes, the cam 34, its lever36 and the vertical rod 37 are set in motion, so that the core 38travels inward and outward relatively to the coil 39. When the condenserplates 33, are out,'-the core 38 is all the way out, which positioncorresponds to a higher-frequency limit of the circuit. When the plates33 are in,

" circuit has selective properties substantially the the core 38 is inthe coil and this position corresponds to a lower-frequency limit. Themovements of the core in relation to the rotation of the condenserplates is governed by the curvature of the cam 34. It is possible tomove the core 38 proportionally to the angular movement of the rotaryplates, in which case the condenser plates may be so shaped as to givethe desired capacity variations. However, it is preferable to employsemi-circular plates producing straightline capacity variations, and toso choose the curvature of the cam 34 that the speed of the core will beaccelerated relatively to that of the rotary plates 33 of the condenser.This train of elements admits of a slow movement of the core 38 whensaid plates are having their initial angular motion, but causes agradual acceleration of the speed of the core 38 which reaches itsmaximum when the plates 32 and 33 coincide. Such combination ofsemi-circular plates with a progressively moving core, eifectuates theproduction of equally wide frequency channels throughout the entirerange of the selector.

Having thus described my invention, what I claim is:

1. A radio-frequency amplifying circuit including a relay device and aselective circuit electrically connected to the output terminals of saidrelay device and having an inductance coil, an external capacity acrosssaid coil, and acompressed magnetic body disposed in the field of saidcoil, said body having insulated magnetic particles and being of suchcharacteristic that the amplifying circuit has selective propertiessubstantially the same as when said magnetic body is wlthdrawn.

2. A radio-frequency amplifying circuit including a relay device and aselective circuit electrically connected to the output terminals of saidrelay device and having an inductance coil, an external capacity acrosssaid coil, and a compressed magnetic body disposed in the field of saidcoil, said body having insulated magnetic particles of such finenessthat said amplifying same as when said magnetic body is withdrawn. 3. Aradio-frequency amplifying circuit including a relay device and aselective circuit electrically connected to the output terminals of saidrelay device and having an inductance coil, an external capacity acrosssaid coil, and a compressed magnetic body disposed in the field of saidcoil, said body being of 'a closedmagnetic type and having insulatedmagnetic particles to maintain the selectance of said amplifying circuitat the 13C desired value.

4. A radio-frequency amplifier including a relay device and a selective.circuit electrically connected to the output terminals of said relaydevice and having an inductance coil, an external 13! capacity acrosssaid coil, and a magnetic body disposed in the field of said coil, saidbody being compressed and having particles insulated by an elasticsubstance and being movable relatively to said coil to vary the periodof said selective circuit l4( while retaining the desired amplification.

5. A radio-frequency amplifier including a relay device and a selectivecircuit electrically connected to the output terminals of said relaydevice, said selective circuit having an inductance 14- terminals ofsaid relay device, said selective cir-' cuit having an inductance coil,an'external capacity and a compressed magnetic body disposed in thefield of said coil, said body having insulated magnetic particles andbeing movable relatively to said coil to vary the period of saidselective cir cuit while retaining the desired amplification.

7. A radio-frequency amplifying circuit including a relay device and aselective circuit electrically connected to the output terminals of saidrelay device, said selective circuit having an inductance coil, anexternal capacity and a compressed magnetic body disposed in the fieldof said coil, said body having insulated magnetic particles and beingmovable relatively to said coil to vary the period of said selectivecircuit while maintaining the selectance of said amplifying circuit atthe desired value.

8. A radio-frequency amplifying circuit including a relay device and aselective circuit electrically connected to the output terminals of saidrelay device, said selective circuit having an inductance coil, anexternal capacity and a compressed magnetic body disposed in the fieldof said coil,

said body having insulated magnetic particles and being movablerelatively to said coil to vary the inductance and the radio-frequencyresistance of said selective circuit simultaneously and in the sameproportion so as to produce substan-' tially uniform amplification andselectance in said amplifying circuit throughout the range ofadjustability.

9. A radio-frequency amplifying circuit including a relay device and aselective circuit electrically connected to the output terminals of saidrelay device, said selective circuit having an inductance coil, anexternal capacity and a compressed magnetic body disposed in the fieldof said coil, said body having insulated magnetic particles and beingmovable relatively to said coil to vary the inductance and theradio-frequency resistance of said selective circuit simultaneously andin the same proportion so as to produce substantially uniform selectancein said amplifying circuit of the order of 30 kilocycles width at halfamplitude throughout a frequency range from 550 to 1500 kilocycles.

10. A radio-frequency amplifier including a relay device and aninductively coupled selective circuit, having an inductance coil, avariable capacity and a compressed magnetic body movable in the field ofsaid coil, said body having insulated magnetic particlesthe movement ofsaid body being correlated with the adjustment of said variable capacityso as to produce substantially uniform'amplification throughout therange of adinstability of said amplifier.

11. A radio-frequency amplifier including a relay device and aninductively coupled selective circuit, having an inductance coil, avariable capacity and a compressed magnetic body movable in the field ofsaid coil, said body having insulated magnetic particles, the movementof said body being correlated with the adjustment of said v riablecapacity so as to produce substantial y constant effective dynamicresistance between the 7 output terminals of said relay device.

12. A radio-frequency amplifier having a relay device, a selectivecircuit including the secondary coil of a transformer, an externalcapacity and a compressed magnetic body movable in the field of saidcoil, said body having insulated magnetic particles, a primary windingof said transformer being connected to the output terminals of saidrelay device, the movement of, said body simultaneously varying thesecondary inductance and the mutual inductance. of said transformer totune said amplifier while maintaining the desired amplification.

13. A radio-frequency amplifier having a relay device, a selectivecircuit including a secondary coil of a transformer, a variable capacityand a compressed magnetic body movable in the field of said coil, saidbody having insulated magnetic particles, 2. primary winding of saidtransformer being connected to the output terminals of said relaydevice, the movement of said body being can-elated with the adjustmentof said capacity and simultaneously varying the secondary inductance andthe mutual inductance of said transformer to produce desiredamplification in said amplifier.

'14. A system including a plurality of radio-frequency amplifyingcircuits, having selective input and interstage circuits arranged incascade, said selective circuits eachhaving an inductance coil, anexternal capacity and a compressed magnetic body disposed in the fieldof said coil, said body having insulated magnetic particles and beingmovable relatively to said coil for tuning said system while maintainingthe selective properties of said system at the desired values.

15, A radio-frequency amplifying circuit including a relay device and aselective circuit electrically connected to the output terminals of saidrelay device and having a low-loss inductance coil, a capacity acrosssaid coil, and a compressed magnetic body disposed in the field of saidcoil, said body having insulated magnetic particles and being of suchcharacteristic that said amplifying circuit maintains its selectiveproperties substantially the same as when said magnetic body iswithdrawn.

16. A radio-frequency amplifier including a relay device and a selectivecircuit electrically connected to the output terminals of said relaydevice, said selective circuit having a low-loss inductance coil, acapacity across said coil, and a compressed magnetic body disposed inthe field of said coil, said body having insulated magnetic particles ofsize small enough to pass through a screen of 300 meshes per inch andbeing movable relatively to said coil to vary the period of saidselective circuit while retaining the desired amplification.

1'7. A system including a plurality of radio-frequency amplifyingcircuits, having selective input and interstage circuits arranged incascade, said selective circuits each having an inductance coil, anexternal capacity and a compressed magnetic body disposed in the fieldof said coil, said body having insulated magnetic particles and beingmovable relatively to said coil for tuning said system while preservingsubstantially uniform amplifying properties in said system.

;/18. A system including a plurality of radiof ziequency amplifyingcircuits, having selective i put and interstage circuits arranged incasade, said selective circuits each having an inductance coil, anexternal capacity and a compressed magnetic body disposed in the fieldof a whereby the variation of the capacity and the movement of the corevary the period of the selective circuit while maintaining theamplification and selectance at the desired value.

WLADIMIR J. POLYDOROFF.

